Titanium(IV)–Salan Catalysts for Asymmetric Sulfoxidation with Hydrogen Peroxide

We report the synthesis and the solution and solid‐phase characterization of several titanium(IV)–salan complexes. The structures of the ligands H 2 sal( R , R ‐chan), H 2 ovan( S , S ‐chan), and H 6 pyr( R , R ‐chan) 4+ · 4Cl – · H 2 O were determined by single‐crystal X‐ray diffraction and their main features are compared and discussed. Single crystals suitable for X‐ray diffraction studies were also obtained for [Ti 4 {sal( R , R ‐chan)} 4 (μ‐O) 4 ] and showed a tetranuclear core bearing μ‐oxo bridges between the titanium atoms and the ligand wrapped around the Ti in a fac‐mer geometry. The Ti–salan complexes were tested as catalysts in the benchmark oxidation of thioanisole with H 2 O 2 as oxidant in organic solvents. High H 2 O 2 conversion and moderate enantioselectivities (up to 51 % ee ) were obtained. In an effort to perform greener catalysis the complexes were tested in selected ionic liquids with several oxidants. [Ti{5‐MeO‐sal( R , R ‐chan)}] was also covalently bound to a polystyrene matrix. tert ‐Butyl hydroperoxide was found to be the most suitable oxidant for use in ionic liquids (100 % conversion vs. 80 % with H 2 O 2 ) and in general the Ti–salan complexes showed better activity in ILs at 25 °C although with reduced induced asymmetry (up to 18 % ee ).